Also, try iStockphoto, put "icon" in the search field. You find there as well 3D images,(you can restrict to only have vectorial results, but I'd look in normal raster, as well..) high quality images, etc. They don't let just any thing to be uploaded: there is a quality control. In some cases you might prefer just purchase a nice 3D image and produce yourself the low res icon sizes, or have someone do it, if the icon sets aren't high res enough, or really like certain image/photo/3D for your icon. The database they have is huge.